Method for spot welding galvanized sheet metal

ABSTRACT

The present invention is directed to an improved method and apparatus for effecting spot welds between superimposed sheets of galvanized metal. In accordance with the method, one of the electrodes is comprised of a consumable tip component, preferably a steel ball, whereby predictable and repeatable welding conditions are effected at each welding cycle. The invention is further directed to a method of effecting spot welds between galvanized sheets which includes the step of interposing between a conductive carrier electrode and the metal surface a consumable metallic increment having reduced contact area adapted to engage the galvanized sheet, the consumable component being compatible with the ferrous substrate beneath the galvanized layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of spot welding, and moreparticularly relates to an improved method and apparatus for effectingspot welds between superimposed galvanized metal sheets.

2. The Prior Art

Spot welding has long been recognized as an effective and economicaltechnique for connecting superimposed sheets of metal. In accordancewith typical spot welding practice, superimposed metal sheets areclamped between arms defining electrodes. Preferably the electrode tipsare pointed and are compressed against registering opposed faces of thesheets.

A high current is applied for a short period of time, resulting inmelting of the metal in the area between the tips and fusing together ofthe sheets in the molten area.

Since welding is effected utilizing low voltages and high amperages,welding can be accomplished only after each of the electrodes is in goodelectrical contact with the metal. The desired effective electricalcontact with the metal is best achieved when the electrodes are sharpand are thus able to penetrate any scale or zinc coatings (in the caseof galvanized iron).

The typical metal employed for spot welding electrodes is copper sincecopper is not compatible with the molten metal formed in the course ofthe welding operation and thus a copper electrode may be readily liftedclear after a weld has been effected. While copper electrodes have theadvantage of incompatibility to the molten material and excellentelectrical conductivity, copper electrodes are disadvantageous in thatthe same are soft, particularly at welding temperatures, and exhibit atendency to mushroom or flatten at the tip area, with the result that aninitially sharp copper electrode which may readily make good contactunder forces in the area of 50 to 80 pounds soon becomes rounded orflattened. As a result of such tendency, it is common practice,particularly where spot welds are to be effected between galvanizedsheets, for the operator to tilt or wiggle the sheet or the electrodesor both to facilitate at least partial penetration to the base metal. Ifthe current is applied without such penetration, the zinc with which theelectrodes are in contact will volatilize, with substantial sparking andspattering and there will be insufficient melting of base metal toeffect a proper connection between the sheets.

To compensate for the mushrooming tendencies described and for coatingof the electrodes by volatilized zinc, it is common practice for weldersto file or redress the points of the electrodes, with resultant reducedproductivity and premature consumption of the electrodes.

A further disadvantage of current spot welding techniques, occasioned inlarge measure by the inability of the welding device to achieveeffective contact with the sheets to be connected, is that one or bothof the electrodes be water cooled, utilizing an open water coolingsystem wherein a continuous supply of cold water is circulated, withresultant water wastage. This type of water cooling is required in partbecause of the excess heat generated by the welding electrodes inburning through the galvanized coating.

It will thus be appreciated that the amount of electricity in the courseof forming a spot weld using conventional equipment, and the consequentcooling requirements, could be greatly reduced if good contact with basemetal were assured and the energy required to reach base metaleliminated.

A further exposition of the problems of spot welding galvanized metalsis contained in U.S. Pat. No. 3,102,190 to Chapleur et al. (Aug. 27,1963) and 3,909,581 to Stone et al. (Sept. 30, 1975), which patentspropose to facilitate welding respectively by bonding powdered metal tothe sheets in the weld area, and by the use of a welding tip alloyedwith chromium, beryllium, zirconium, cadmium, silver, gold or platinum.

SUMMARY

The present invention may be summarized as directed to a method andapparatus for effecting spot welds between galvanized sheets, makingefficient use of energy, and minimizing of the undesirable effectsheretofore associated with welding galvanized sheets, namely, theformation of incomplete welds, arcing, spattering, and frequentattention to the welding tip.

In accordance with the method of the invention, one of the weldingelectrodes is comprised of a consumable material, preferably a steelball, which defines the electrode tip, the ball being melted into theweld during each welding cycle and replaced by a further ball for thenext cycle. By this means there is assured a predictable small contactarea between the welding electrode and the galvanized sheet, such smallcontact area simplifying penetration to base metal. Since predictablepenetration is provided, arcing, spattering and volatilization of zincare minimized and substantially less electrical energy is required to beexpended to effect a satisfactory weld.

Additionally, since much of the heat build-up in welding occurs withinthe steel ball and the steel ball is left to flow into the material ofthe weld, heat build-up in the welding electrode which carries the ballsis minimized, simplifying the cooling operation of the electrode.

It has been determined that through the use of consumable electrode tipsin accordance with the invention, a 10 kilovolt amp (kva) power pack maybe used for an operating cycle of one tenth second to achieve fullysatisfactory results in spot welding galvanized sheets of 18 to 24gauge. In contrast, 20 to 40 kva power packs are customarily used forsuch operations.

A further advantage inhering in the method and apparatus of the presentinvention is that, due to the continuously optimized contact achieved bythe consumable electrode tip, it is feasible to employ as the other orground electrode a member having a relatively large, flat contactingarea. The use of such large contact area is desirable in that it acts inthe manner of a heat sink at the outer surface of the lowermost of thetwo superimposed sheets, whereby the discoloration and burninginherently present with welds formed in galvanized materials iseliminated or minimized.

Accordingly, it is an object of the present invention to provide animproved method of spot welding galvanized sheets.

A further object of the invention is the provision of a method of thetype described wherein galvanized sheets may be spot welded togetherwith minimal spattering, flash and zinc volatilization.

A still further object of the invention is the provision of a method ofthe type described wherein energy employed in welding is used in ahighly efficient manner, enabling lower capacity welding equipment toform welds of strength equivalent to that heretofore achieved only withhigher capacity devices.

It is a still further object of the invention to provide a method ofspot welding of the type described wherein discoloration and burningheretofore associated with welds formed in galvanized metals isminimized or eliminated.

A still further object of the invention is the provision of a spotwelding method for use particularly in the connection of galvanizedmetal sheets, employing a consumable metallic electrode tip, the tipbeing replaced following each cycle, whereby the contact area at eachwelding cycle is optimized for the particular welding condition.

A further object of the invention is the provision of a method of thetype described which eliminates the necessity for sharpening orreplacement of the main electrode member, as is necessary withconventional spot welding techniques.

Still a further object of the invention is the provision of an improvedmethod for spot welding galvanized sheets which includes the step ofinterposing a consumable metal electrode tip, preferably a sphericalmember, in the current path, the consumable member being interposedbetween the surface of one of the two sheets to be spot welded and awelding electrode.

Another object of the invention is the provision of a spot weldingapparatus particularly adapted for the welding of galvanized metalsheets.

Still another object of the invention is the provision of an apparatusof the type described, and including means for automatically feeding toan electrode a spherical or other shaped consumable member having afirst and larger contact area in electrical connection with anelectrode, and a second and reduced contact area adapted to be engagedagainst the surface of the sheets to be connected, the reduced contactarea preferably being of such small size as to enable contact to beeffected with the base metal responsive to the pressures normallyexerted in the course of forming a spot weld, e.g. 50 to 80 pounds.

Still a further object of the invention is the provision of a spotwelding apparatus particularly adapted to the spot welding of galvanizedsheets comprising, in combination, a fixed electrode, an electrodemovable toward and away from the fixed electrode, means for sequentiallyfeeding to the movable electrode consumable metal members, such asspheres, means for urging the movable electrode toward the fixedelectrode, and means for passing a welding current between theelectrodes with the consumable electrode member forming a component ofthe circuit path, whereby a spot weld is formed and the material of theconsumable member is deformed and flows into the weld.

Still a further object of the invention is the provision of an apparatusof the type described wherein the movable electrode includes arelatively large surface area engaged with a comparable surface area ofthe consumable member as contrasted with the area of the consumablemember engaged against the metal sheets to be welded.

Still further it is an object of the invention to provide an apparatusof the type described wherein a consumable member is automatically fedto the movable electrode member to define a tip following each weldingcycle.

To attain these objects and such further objects as may appear herein orbe hereinafter pointed out, reference is made to the accompanyingdrawings, forming a part hereof, in which:

FIGS. 1, 2 and 3 are vertical sectional views, diagrammatic in nature,through the welding head of a welding apparatus showing sequentially theloading of a consumable welding tip into position within the weldingelectrode, advancement of the composite electrode into position foreffecting a weld, and the position of the parts after a weld has beencompleted.

Referring now to the drawings, there is diagrammatically disclosed awelding apparatus and method particularly adapted for the spot weldingof superimposed galvanized sheets.

Illustratively, where it is desired to connect by spot welding an uppergalvanized sheet 10 having galvanized zinc coatings 10' and a lowergalvanized sheet 11 having galvanized coatings 11', the superimposedsheets are placed atop a ground electrode 12. Whereas the groundelectrode 12 in the illustrated embodiment is disclosed as having aplanar upper surface 14, which surface may be round, square, etc. inplan, it will be readily recognized that the lower electrode may besharpened or pointed so as to facilitate making contact with the basemetal covered by the thin zinc skin which characterizes galvanized sheetmaterial.

The use of a flat electrode, while advantageous in that marring orburning at the lowermost surface of the under sheet is minimized,requires the use of higher capacity welding apparatus.

An upper electrode assembly 15 is disposed in spaced relation to theelectrode 12. While the electrode assembly 15 is shown to be verticallyspaced above the electrode 12, it will be understood that positioning ofthe noted electrodes is immaterial to the satisfactory operation of theapparatus.

The electrode assembly 15 is comprised of a tubular guide sleeve 16having adjacent its lower end 17 a magnet member 18 on the inner surface19.

The magnet member 18 functions as a detent, releasibly supporting thelowermost consumable welding member 20 at a position adjacent the lowerend 17 of the electrode assembly 15. The consumable members 20preferably comprise spherical steel balls. While the spherical shape isadvantageous in that steel balls are inexpensive to fabricate and may bereadily fed to the electrode tip without orientation, the invention isnot to be construed as limited to such shape.

Important attributes of the configuration of the consumable member 20are that they present a reduced area contact with the upper galvanizedsheet 10 when pressed against the surface thereof and that the interfitbetween the consumable member and the active electrode 21 provides alarge contact area at the interface of the consumable member and theelectrode. Additionally, it is desirable that the consumable member beof ferrous material since, due to the magnetic attractable properties,such material facilitates handling of the members. Also, ferrousmaterials are compatible with the weld and will flow and interfuse withthe material of the sheets which are melted in the course of welding.

The consumable members 20, 20' may be guided to the welding assembly 15in the tubular conduit 22 extending from a storage hopper (not shown).

In the "ready" position illustrated in FIG. 1, the lowermost ball 20 isshown to have descended the guideway 22 into retained position on themagnetic member 18, which may be recessed as at 18' to minimize thechance of the member 20 dropping free from the lower end of theelectrode assembly 15.

In the formation of the weld, the active electrode 21, which ispreferably encased in a heat resistant insulation sleeve 23, is shifteddownwardly toward the superimposed sheets 10, 11. The base portion 24 ofthe active electrode 21 is enlarged and terminates in a downwardlydirected parti-spherical or cup-shaped receiver pocket 25 having amagnet member 26 embedded therein. The radius of curvature of thecup-shaped portion 25 is formed to correspond with the radius ofcurvature of the consumable members 20 so as to provide a maximum areaof contact at the interface between the electrode and the consumablemember.

In order to form a weld, the active electrode 21 is shifted downwardlypast the magnet 18, which downward movement will result in the lowermostconsumable member 20 being removed from magnet 18 and retained withinthe cup member 25 by the magnet 26. Further downward movement of theactive electrode 21 will cause the consumable member 20 to press tightlyagainst the upper surface of the sheet 10 with a desired downward force.

By way of example and without limitation, the members 20 may becomprised of 1/8" diameter steel balls. The hardness of the steel is notparticularly critical, it being oserved that virtually any steelcomposition will be of greater hardness than the zinc coatings 10', 11'applied to the metal sheets 10, 11. Preferably, the balls are selectedof a material which melts at a temperature comparable to that of thegalvanized sheets. A downward force of from about 50 to 80 pounds hasbeen found satisfactory with the use of 1/8" diameter balls to achievethe desired degree of electrical contact with the upper sheet 10.

With the parts positioned as shown in FIG. 2, the apparatus is incondition for formation of a spot weld. It will be noted that theconsumable member or ball 20' is held from descending the guideway 22 bythe side surface of the insulating sleeve 23. Obviously the diameter ofthe chamber at the lower end of the electrode assembly 15 is coordinatedwith the size of the balls such that only a single ball can occupy thechamber at any given time, and (in the raised position of the electrode)the ball positioned within the chamber and retained on the magnet 18operates to block downward movement of the next adjacent ball 20'.

A welding current is then passed through active electrode 21, ball 20,sheets 10 and 11, and to ground electrode 12, while a downward force iscontinuously applied. As noted, utilizing a 1/8" diameter steel ball, a10 kva power pack may be employed for a welding cycle of 1/10th secondto achieve an effective spot welding of galvanized sheets of thicknessvarying from 18 to 24 gauge. The welding assembly 15 incorporates a stopmechanism (not shown) which functions to prevent the lowermost end 27 ofthe electrode from actually contacting the uppermost sheet 10.

After formation of the weld, FIG. 3, the sleeve 23 carrying electrode 21is raised to its upper limiting position, FIG. 1, whereupon the nowlowermost ball 20' will be fed gravitationally to the positionpreviously occupied by the ball 20.

The sheets 10, 11 will be shown to be securely spot welded together, theappearance of the formed weld 28 normally differing slightly from theappearance of a conventional spot weld in that the weld in accordancewith the invention includes a slight protuberance or projection 29whereas a conventionally formed spot weld generally includes a slightrecess in the area engaged by the electrode during the formation of theweld.

In contrast to the 1/10th second weld cycle used in accordance with theinvention with a 10 kva power pack to spot weld galvanized sheets of thethickness range noted, conventional spot welding procedures require theutilization of 1/2 second welding cycles using 20 to 40 kva power packsto assure connection of the sheets in situations where initial contactwith the sheets was imperfect. It will thus be observed that substantialenergy savings are realized through the use of the method and apparatusof the present invention, and also, the initial investment in weldingequipment is lessened.

A further advantage in the use of the present invention is that the heatbuild-up in the electrode is reduced since much of the heat isconcentrated in the consumable member and such member remains as a partof the spot welded sheets. Thus, where a cooling system is employed forreducing the temperature in the active electrode, the capacity of suchsystem may be substantially less than is the case in conventional spotwelding methods.

The weld is effected with minimal arcing and spattering, and where aflat electrode is employed on one surface, discoloration or burning ofthe sheet adjacent the ground electrode is minimized.

Spot welds may be effected with a high degree of predictability sincethe condition of the welding tip formed by the consumable member isidentical in each instance.

The consumable member is preferably selected so as to melt in the samegeneral temperature range as the sheets to be welded.

While the device for practicing the instant method has been illustratedas an automatic feeding device wherein the consumable members aremechanically fed to the welding electrode during each welding cycle, itwill be recognized that many of the advantages of the present inventionmay be achieved utilizing a manual application of the consumable weldingtip to the active electrode.

Central to the concept of the instant invention is the use of aconsumable member which functions as a replaceable welding tip, thedepth of the consumable member, i.e. the distance from the point ofcontact with the metal sheet and the welding electrode, being minimal,i.e. not substantially greater than the maximum cross-sectional area ofthe consumable member, the contact area between the electrode andconsumable member being at a maximum, i.e. about 20% or more of thesurface area of the metal member, and the contact area between theconsumable member and sheet being at a minimum to facilitatepenetration.

While the consumable member is preferably selected of a material suchthat the same will melt and flow into the weld in the course of theformation thereof, certain of the benefits of the invention may beachieved where the consumable member, while having the other attributeshereinabove noted, is not completely deformed in the course of formationof the weld.

The apparatus preferably includes a stop means (not shown) for limitingmovement of the electrode 21 toward the sheets to prevent direct contactof the sheets and electrode. Such stop means may take the form, by wayof example, of a thin insulating rim fixed to the electrode surroundingthe tip and projecting a slight distance therebelow.

While the apparatus of the invention has been illustrated utilizing amagnet in the electrode supporting the spherical consumable members, itwill be apparent that other support means, such as a friction support,may be suitably employed.

It is also feasible to employ an opposed pair of electrodes, each havingconsumable tip members, for the formation of the spot weld, althoughsuch arrangement will not normally be required.

While the principal utility for the present invention is considered toreside in the spot welding of galvanized sheets, it will be appreciatedthat the underlying concept may be employed in the connection of metalsheets other than galvanized or ferrous sheets.

From the foregoing disclosure it will be readily recognized by thoseskilled in the art that modifications and variations may be made withoutdeparting from the spirit of the invention. Accordingly, the inventionis to be broadly construed within the scope of the appended claims.

Having thus described the invention and illustrated its use, what isclaimed as new and is desired to be secured by Letters Patent is:
 1. Themethod of connecting two superimposed sheets of galvanized coatedferrous metal by welding which comprises the steps of contacting an areaof an outer surface of one of said superimposed sheets with a firstelectrode, contacting an outer surface of the other of said superimposedsheets at an area in registry with said area contacted by said firstelectrode with a consumable ferrous metal member having a greaterhardness than the galvanized coating to provide the necessarypenetration to the ferrous metal of said superimposed sheets, saidconsumable metal member having a reduced contact area engaging saidsurface of said other sheet, contacting a major portion of the surfaceof said consumable metal member with a conductive carrier electrode, andcontinuously urging said carrier electrode toward said first electrodewhile simultaneously passing an electrical current between saidelectrodes sufficient substantially to deform said consumable metalmember and to interfuse contacting portions of said sheets disposedbetween said member and said first electrode.
 2. The method inaccordance with claim 1 wherein said consumable metal member comprises asphere and said carrier electrode includes a parti-spherical pocketportion conforming to the surface of said sphere.
 3. The method inaccordance with claim 2 and including the step of automatically feedinga consumable metal member to said carrier electrode responsive tomovement of said carrier electrode away from said sheets.
 4. The methodin accordance with claim 2 wherein said sphere is comprised of ferrousmetal having a melting temperature generally equal to the meltingtemperature of said sheets.
 5. The method in accordance with claim 4 andincluding the step of magnetically supporting said sphere in said pocketportion of said tip.
 6. The method of spot welding two superimposedsheets of galvanized coated ferrous metal which comprises the steps ofcontacting an area of an outer surface of one of said superimposedsheets with a first electrode, contacting an outer surface of the othersaid superimposed sheets at an area in registry with said firstelectrode with a consumable metal member shaped to define substantialpoint contact with said other sheet, said consumable metal member havinga greater hardness than the galvanized coating to provide penetration ofthe galvanized coating to the ferrous metal of said superimposed sheets,engaging areas of said consumable metal member larger than the areacontacting said other sheet with a second electrode, biasing said secondelectrode toward said first electrode while simultaneously passing anelectrical current through a conductive path including said secondelectrode, said consumable member, said sheets and said electrode,thereby to interfuse said contacting portions of said sheets andportions of said consumable metal member, and substantially completelydeform said consumable metal member.
 7. The method in accordance withclaim 6 and including the step of interrupting movement of said secondelectrode toward said first electrode after said substantial deformationof said consumable member and in advance of contact between said secondelectrode and said other sheet.
 8. The method in accordance with claim 7wherein said consumable member comprises a ferrous sphere.
 9. The methodin accordance with claim 8 and including the step of magneticallysupporting said sphere to said second electrode.
 10. The method ofconnecting by spot welding two superimposed sheets of metal havinggalvanized surface coatings which comprises the steps of contacting thearea of an outer surface of one of said superimposed sheets with a firstelectrode, penetrating the galvanized coating surface of the other ofsaid superimposed sheets at an area in registry with the first area witha consumable metal member formed of a material compatible with at leastthe other of said superimposed sheets, said consumable metal memberhaving a greater hardness than the galvanized coating and being meltableat substantially the same temperature as said sheets and having areduced contact area engaging said surface of said other sheet,contacting a major portion of the surface of said consumable metalmember with a conductive carrier electrode and continuously urging saidcarrier electrode toward said first electrode while simultaneouslypassing an electric current between said electrodes sufficientsubstantially to deform said metal member and interfuse contactingportions of said sheets disposed between said member and said firstelectrode.
 11. The method in accordance with claim 10 wherein saidconsumable metal member comprises a sphere and said carrier electrodeincludes a parti-spherical pocket portion conforming to the surface ofsaid sphere.